Choosing Rootstocks
Rootstocks influence apple trees in many ways: their growth rate, final size, early cropping, fruit size, yield potential and response to nutrition. Rootstocks vary in their susceptibility to insects and diseases, tolerance of soil conditions, and compatibility with various scions. Rootstocks also vary in their ability to withstand cold winter temperatures, as well as freeze-thaw cycles during the dormant season. So it is important to match the correct dwarfing rootstock to your soil, site and cultivar so your trees grow the appropriate size and produce high early yields of large, quality fruit.
Most high density orchards require a dwarf tree similar to the size produced by Malling 9 (or M.9) rootstocks. M.9 was selected in France in 1828, and later developed by the East Malling Research Station around 1912. It has been widely used in Europe, North America and world-wide for many years. M.9 produces a compact tree that produces high early yields, with improved fruit size, but it has several potential problems. M.9 is very susceptible to fire blight, and increases the susceptibility of the scion to fire blight. It tends to produce root suckers, (making it easy to propagate in the nursery), but this characteristic makes it vulnerable to fire blight entry into the rootstock. M.9 is also susceptible to apple replant disease, crown rot, and woolly apple aphids. Recent observations indicate that some strains of M.9 are less winter hardy, especially the popular NAKB 339 strain.
Strains of M.9: Since the introduction of the Malling series almost 100 years ago, strains have been selected for better performance and/or through virus removal. In England, the East Malling-Long Ashton program removed viruses to produce M.9 EMLA, which is more vigorous than virus-infected M.9. Pajam 2 was selected in France to have more vigour. Holland developed M.9 T337 through its NAKB plant health program, and although T337 is likely the most commonly planted strain, recent reports indicate concerns about its hardiness. Nic 29 or Cepiland was also virus-cleaned and selected in Belgium , and is reported to have improved vigour than "dirty" M.9.
Several other rootstock programs have developed dwarfing rootstocks suitable for today's high density planting. These three programs have or are currently providing rootstocks of interest:
- Cornell-Geneva series: Developed in New York with emphasis on yield efficiency, fire blight resistance, replant tolerance, as well as cold hardiness and resistance to Phytophthora and woolly apple aphid.
- Vineland series: Developed in Ontario by crossing cold hardy crabapples with Robusta and M.9 to select fully-dwarfing rootstocks with winter hardiness and fire blight resistance.
- Budagovsky series: Developed in the Soviet Union for winter hardiness, yield efficiency and disease resistance. Bud.9 or B.9 has been planted most widely.
It takes a long time to properly evaluate a rootstocks performance, but some testing has been done under Ontario conditions or in neighbouring areas. We are collating information from many sources to learn more about these new rootstocks and the suitability for Ontario orchards.
